30 L is about as small as I can go for a two or three day trip.
I've adopted a bit of a different viewpoint on my pack sizing ....
My summer Bag takes up about 8 L, my Clothes take up about 8 L with a high loft pullover, I reserve 2 L for mic. gear such as first aid, headlamp, etc, 2 L for my Gatewood cape, 2 L for my pad, and 2 L for my A6 Adventure Bivy.
This takes up 24 L of volume.
Water and Food remain ... 2 L of water added to a 33 L pack leaves 7 L for food and cooking gear ... 0.6 L for cup, spoon, windscreen, and stove, and about 400 cu in for food, or about 3 and a half days worth ( a little over 2.1 L per day, food and drink mixes).
A 40 L pack will take me out to about 5 days with the extra food, TP, fuel, and a book tossed in.
I typically carry a 62 L pack for trips over 5 days.